COMPLEMENT SYSTEM
Dr. Iram Sohail
Assistant Professor
Pathology
College Of Medicine
Majmaah University
OBJECTIVES
• Define complement & give its clinical
importance
• Explain the biological effects of complements
• Review the diseases due to complement
deficiency
Complement system
Definition
• The complement system consists of plasma
proteins that plays an important role in host
defense (immunity) and inflammation.
Biological effects & clinical importance
1. Opsonization
• Different complement proteins coat
(opsonize) particles, such as c3b coats
microbes for phagocytosis and destruction of
microbes
2. Contribute in inflammation
• Complements also contribute in the
mechanism of inflammation by increasing
vascular permeability and Chemotaxis of
WBCs.
• Complements also generate membrane attack
complex (MAC) that causes formation of holes
in membranes of microbes.
• Most of the complements (C1 to C9) are
present in inactive form in plasma.
• This activation starts with the activation of C3
via 3 pathways
1. Classical pathway
• It starts (stimulates) by the fixation of C1 to
antigen-antibody complex.
2. Alternative pathway
• It starts with bacterial endotoxins.
3. Lectin pathway
• These entire pathways end with the formation
of C3 convertase, and this enzyme converts C3
into C3a and C3b.
• Eventually C5 convertase formed and it
converts C5 into C5a and C5b and finally C6 to
C9 synthesized.
Vascular effects
C3a and C5a
– (Also called Anaphylatoxins)
– Causes increased vascular permeability &
vasodilation by inducing mast cells to release
histamine.
C5a
• Causing
– release of inflammatory mediators from
neutrophils & macrophages
– leukocyte activation
– leukocyte adhesion
– leukocyte Chemotaxis
• C3b
– Opsonization & Phagocytosis
(coat microbes and make it easy for phagocytosis)
DISEASES DUE TO COMPLEMENTS DEFICIENCES
• Systemic lupus erthromatosus (SLE)
• Infections
• Glomerulonephritis
• Paroxysmal nocturnal hemoglobinuria
• Hereditary angioedema
• Atypical hemolytic uremic syndrome